THE MINERALIZATION AT CLIFTON-MORENCI 


Louis E. Reber, Jr 

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Economic Geology Publishing Company 














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THE MINERALIZATION AT CLIFTON-MORENCI. 


[Reprinted irom Economic Geology, Vol. XI., No. 6, August-September, 1916. | 


THE MINERALIZATION AT CLIFTON-MORENCI. 

Louis E. Reber, Jr. 

SCOPE OF PAPER. 


Scope of Paper . 5 2 ^ 

Sources of Material . S 2 ^ 

General Geology . 5 2 9 

Mineralization . 533 

Detailed Description of Rock Alteration . 534 

Processes of Alteration . 55 2 

Interrelation of Processes of Alteration . 564 

Origin of Mineralizing Solutions . 570 


This paper presents a study of the mineralization associated 
with the intrusive stock to which are attributed the ore-deposits 
of the Clifton-Morenci district. From detailed microscopic and 
field study results are brought out bearing on the nature of the 
various phases of mineralization or rock alteration, and particu¬ 
larly on their mutual relationship and relation to the igneous in¬ 
trusive. Contact alteration is contrasted with hydrothermal alter¬ 
ation, and the two types of hydrothermal alteration, sericitiza- 
tion and propylitization, are compared; lastly evidences bearing 
on the source of solutions effecting the mineralization are dis¬ 
cussed and interpreted. 

SOURCES OF MATERIAL. 

The writer spent four months in the Clifton-Morenci district 
in the summer of 1915, and during the following winter some 
time was devoted to the study of a very comprehensive suite of 
specimens there collected, including numerous thin and polished 
sections of this material. 

The Professional Paper by Lindgren 1 which appeared in 1905 
includes practically all the published results of geological study 
of this district. This work necessarily forms a starting point 

5 2 8 


Gin 

tote w ^ 


U. S. G. S. Prof. Paper 43. 










MINERALIZATION AT CLIFTON-M O REN Cl. 529 

for any later investigation'. The general descriptive and ex¬ 
planatory matter here summarized is to be found in more detail 
in that paper. 

At the time Lindgren completed his work on the Clifton-Mo- 
renci district it would have been difficult to add anything to that 
detailed and exhaustive study. Since then, however, a large 
amount of development work has been done in the mines; also 
the results of investigations in other districts have suggested new 
possibilities in regard to the relations which may exist between 
the processes of magmatic differentiation, ore-deposition, and 
rock-alteration. Therefore it is believed that further study of 
this relationship as exemplified in the Clifton-Morenci district is 
warranted. 

GENERAL GEOLOGY. 

The Clifton-Morenci district lies in the southern part of Green¬ 
lee county in southeastern Arizona, close to the New Mexico 
line and about 150 miles north of the international boundary. It 
belongs to that group of districts to which in recent years the 
name “porphyry copper camps” has-been popularly applied. 
Like many geological terms which come into vogue in mining 
circles, this term has a somewhat indefinite meaning. At Morenci, 
however, it has a more exact significance than in certain other 
localities. That is to say: the bulk of the ore minerals occur in 
disseminated particles or veinlets in porphyry, though usually as¬ 
sociated with some larger veins; and the ore-bodies owe their 
commercial value to chalcocitization by the processes of second¬ 
ary enrichment. 

The country rocks of the region are pre-Cambrian granite, 
Paleozoic and Cretaceous sediments, and the younger intrusives. 
Fig. 28 is a geological map showing the areal distribution, and 
Fig. 29 is a columnar section showing diagrammatically the thick¬ 
ness and sequence of the various formations. 

The Paleozoic system which is apparently comformable 
throughout, is separated from the underlying pre-Cambrian 
granite by a great unconformity. It comprises about 200 feet 
of sandstone or quartzite of Cambrian age, 380 feet of Cambro- 


530 


LOUIS E. REBER, JR. 


Ordovician limestone, 175 feet of impure limestone and calcare¬ 
ous shale which is 'believed to be of Devonian age, and 170 feet 


;"T'i SCALE r M ' LES r " i 



of Mississippian limestone. At the top of this series is an im¬ 
portant erosional disconformity, though there is slight structural 











MINERALIZATION AT CLIFTON-MORENCI. 


531 


unconformity between this system and the overlying formation. 
A banded shale and sandstone formation of early Cretaceous age 


RECENT ALLUVIUM' 


GILA CONGLOMERATE 


PINKARD FORMATION 
175 ' 

DISCONFORMITY 
MODOC LIMESTONE 


MORENCI FORMATION 
175 ' 


LONGFELLOW 

LIMESTONE 

380 ' 


CORONADO QUARTZITE | 
200' 


• UNCONFORMITY 
PRE-CAMBRIAN GRANITE I 


Fig. 29. Columnar Section for the Clifton Morenci District. 


thickness of 175 feet of this has been preserved below the present 
is the sole representative of the Mesozoic system. A maximum 
erosion surface. 

The sediments and underlying pre-Cambrian granite have been 
intruded by a mass of granite-monzonite-diorite porphyry which 
is exposed over an area extending about seven miles in a north¬ 
east south-west direction. The rock is granite porphyry in the 
northeast and diorite porphyry in the southwest, with a monzo- 
nitic phase intermediate. Associated with the intermediate or 

















532 


LOUIS E. REBER, JR. 


monzonitic phase of the porphyry is a dark rock which has been 
found in several mines, but not extensively on the surface. It 
was not exposed when Lindgren was in the district, and Tovote 1 
refers to it as diabase, as tiny apatite needles sometimes give it a 
diabasic appearance. Microscopic study has shown this dark ma¬ 
terial to include two rock types: one, composed largely of finely 
felted green mica, pyrite, and apatite, believed to represent a basic 
segregation in the porphyry (fig. 3) is minutely intruded by the 
other, a quartz-orthoclase-micropegmatite (fig. 7), the dark ap¬ 
pearance of which is due to an unusual brown pigment in the 
feldspar. 

The main intrusive mass is classed as a stock, though some 
laccolithic bodies extend to the southwest. The intrusion of 
this porphyry stock is held responsible for the origin of the min¬ 
eralizing solutions instrumental in forming the copper ores. 

Outside the main mass of porphyry are numerous dikes of the 
same material, disposed more or less radially. With these dikes 
there are a few diabasic ones which are known to cut the por¬ 
phyry, though they probably belong to the same general period of 
intrusion. 

Some time after the intrusion of the diabase and before the 
great outpouring of the late Tertiary volcanics, which cap the 
hills of the outlying districts, the area was affected by important 
normal faulting which has profoundly influenced the topography 
and structure of the district. 

The pre-Cambrian granite, the Paleozdic and Cretaceous sedi¬ 
ments, the porphyry, and the diabase, have all been affected by 
the mineralizing solutions to a greater or less extent. The im¬ 
portant ore-bodies of the early days of the district were found in 
the Paleozoic sediments, while at the present time it is the dis¬ 
seminated ore in the porphyry which is significant. In 1914, 
97 per cent, of the ore mined by one of the larger companies 
was concentrating ore from the porphyry which averaged 2.8 
per cent, copper. 

la Min. and Sci. Press Vol. 101, p. 777, 1910. 


MINERALIZATION AT CLIFTON-MORENCI. 


533 


MINERALIZATION. 

The term mineralization is somewhat indefinite and has been 
subject to varied usage. Probably the most logical conception 
of its meaning is the development of secondary minerals in which 
an important factor is the contribution of material from a source 
independent of the mineralized rocks. This does not come very 
far from limiting it to the work of heated solutions. In this 
paper the term mineralization will be used exclusively to refer to 
the vein filling, and hydrothermal alteration or other metaso- 
matic changes accomplished by the heated solutions associated 
with the porphyry. Thus the rock alteration for which the por¬ 
phyry intrusive is responsible is a phase of this mineralization. 
The distinction between the definition here chosen and another 
equally justified by usage which limits mineralization to the de¬ 
velopment of ore minerals, notably sulphides, should be kept in 
mind. 

In connection with the study of the mineralization the chief 
problem involves the separation and classification of minerals ac¬ 
cording to their genetic occurrence, the recognition of the factors 
which have determined certain mineral associations, and the evalu¬ 
ation of these factors as to their influence on ore-deposition. The 
first distinction to be made is that between the primary rock-form¬ 
ing minerals and those developed by processes of secondary alter¬ 
ation. In further classifying the minerals of the second group 
it is first necessary to distinguish between differences due to the 
nature of the solutions acting, and those due to conditions of 
deposition or differences in the material acted on. In the classi¬ 
fication according to the differences in the agencies of metamor¬ 
phism, at least four types are to be considered, namely: contact 
alteration (pneumatolytic), hydrothermal alteration (sericitic and 
propylitic), weathering (in zone of oxidation), and the work of 
the solutions of secondary sulphide enrichment (meteoric solu¬ 
tions under deoxidizing conditions). 

Minerals deposited in open spaces, chiefly as fissure filling are 
to be distinguished from those developed by alteration and re¬ 
placement. 


534 


LOUIS E. REBER, JR. 


The different materials which influence the nature of the min¬ 
eralization are the different rock formations of the district, namely, 
the igneous rock types, white porphyry, diorite porphyry, green 
mica rock, micropegmatite, diabase, and granite; and the sedi¬ 
mentary rock types, shale, limestone, and quartzite or sandstone. 

DETAILED DESCRIPTIONS OF ROCK ALTERATION. 

Granite Porphyry .—The fresh granite porphyry shows rather 
numerous white feldspar phenocrysts of various sizes (from 2% 
mm. down) in a fine grayish white groundmass. Tiny greenish 
spots are numerous, and small pyrite grains are not rare. There 
are usually some small quartz phenocrysts and occasional large 
bipyramidal ones (5 mm. across) which are the most character¬ 
istic feature of the rock. Weathered specimens are very similar 
to those to be described under monzonite porphyry. 

Typically, the granite porphyry is intensely altered and is com¬ 
posed of sericite, quartz, and pyrite. With the pyrite are micro¬ 
scopic quantities of chalcopyrite. The sericite usually forms an 
extremely fine felt (fig. 2) though where associated with much 
secondary quartz some of it may be relatively coarse grained 
(fig. 1). The abundance of both the quartz and the pyrite 
varies greatly in different specimens. The pyrite occurs dissemi¬ 
nated through the rock or in association with small veinlets of 
quartz which are of common occurrence. Pyrite individuals 
showing good crystal outlines are not rare. The quartz occurs 
chiefly in the groundmass and in the veinlets, while the feldspar 
phenocrysts are usually more or less delimited by areas of rela¬ 
tively pure sericite, though in the most thoroughly altered phase 
this distinction may be entirely lost. 

In the thin sections of the freshest specimens feldspar pheno¬ 
crysts, usually about 1 mm. in diameter or less, occupy about Ve 
of the area. These feldspars are chiefly albite. The large bi¬ 
pyramidal quartz phenocrysts as well as the smaller quartzes are 
of rare occurrence in comparison with the feldspar phenocrysts. 
Small biotite phenocrysts are also sparingly present. The ground- 
mass is a fine microgranitic aggregate of quartz and feldspar. 


MINERALIZATION AT CLIFTON-MORENCI. 


535 


Sericite occurs sparingly in the groundmass and somewhat more 
abundantly in the feldspar phenocrysts. The biotites are either 
merely bleached or altered to chlorite. 

The classification by Lindgren 2 as a granite porphyry is based 
on a partial analysis which gives: Si 0 2 , 69.13 per cent.; CaO, 
0.22; Na 2 0 , 3.01; K 2 0 , 3.94, which is equivalent to 23.5 per 
cent, orthoclase molecule, 25.1 per cent, albite molecule, and 1.1 
per cent, anorthite molecule. 

Aside from those produced by weathering the typical secondary 
minerals of the granite porphyry are: sericite, chlorite, quartz, 
pyrite, chalcopyrite, chalcocite, and kaolin. The sericite, chlorite, 
quartz, and pyrite are typical of hydrothermal alteration, while 
the chalcocite and kaolin are ascribed to the processes of sec¬ 
ondary enrichment. 

The minerals of the zone of weathering to be discussed under 
monzonite porphyry, occur similarly in the granite porphyry. 

Monzonite Porphyry .—In general the monzonite porphyry is 
only distinguished from the granite porphyry by the absence of 
the large bipyramidal quartz-phenocrysts and the scarcity of any 
quartz phenocrysts, though it sometimes has a greenish appear¬ 
ance as the dioritic type is approached. 

The weathered porphyry shows yellow and red-brown colors 
with more or less black staining which make the outcrops pic¬ 
turesque elements of the scenery. The yellow-brown tinge dis¬ 
tinguishes it from the granite and quartzite. 

The sericitized phase of the monzonite porphyry is most typical 
and cannot be distinguished from the similarly altered granite 
porphyry (fig. 2). 

In the thoroughly weathered specimens kaolin is usually abun¬ 
dant and sericite is less conspicuous under the microscope. Opaque 
limonite or hematite spots are of common occurrence. Pyrite 
may be removed leaving clean-cut cavities. Sometimes these are 
filled with opal which may have a limonite rim. Where chalcocite 
occurs it is usually associated with malachite and sometimes cu¬ 
prite. The cuprite and malachite radiate outward from the chal- 

2 U. S. G. S. Prof. Paper 4 3 , 1915 


536 


LOUIS E. REBER, JR. 


EXPLANATION TO PLATE XIX. 

Fig. i. Altered granite or monzonite porphyry. Sericite felt with con¬ 
spicuous quartz. Crossed nicols. 30 diameters. 

FiG. 2. Altered granite or monzonite porphyry. Typical sericite felt. 
Crossed nicols. 30 diameters. 

Fig. 3. Green mica rock showing resemblance to sericite felt. Crossed 
nicols. 30 diameters. 

Fig. 4. Green mica rock showing peculiar pyrite form and apatite rods. 
Ordinary light. 30 diameters. 


Plate XIX 


Economic Geology. Vol. XI 



Fig. 3 


Fig. 4 









MINERALIZATION AT CLIFTON-MORENCI. 537 

cocite from which they were formed and occupy a network of 
ramifying veinlets. Where both occur, the cuprite stays rela¬ 
tively near the chalcocite. Malachite may develop where other 
evidences of weathering are lacking. Chrysocolla is found fur¬ 
ther away from the surface than cuprite or the copper carbonates, 
and bunches of native copper are also found at relatively great 
depths though usually associated with more or less ferruginous 
oxide material. 

A relatively fresh specimen of monzonite porphyry is described 
by Lindgren as follows: 

“ Under the microscopic orthoclase, albite, and oligoclase, with an oc¬ 
casional crystal of labradorite are shown to be present. The plagioclase 
feldspars often show well developed zonal structure. The groundmass 
is coarsely microcrystalline, consisting of quartz and non-striated feld¬ 
spar grains with occasional octahedrons of magnetite. Sericite is pres¬ 
ent in the feldspars while chlorite has formed from the biotite; a little 
secondary epidote also occurs.” 

Thus the fresh rock is a porphyry with phenocrysts of oligoclase, 
albite, orthoclase, biotite, and labradorite, in about the order of 
abundance named, in a micro-crystalline groundmass of quartz 
and feldspar. 

Chemical analysis gives the following approximate mineral 
composition with the feldspar expressed molecularly: 


Percentage of Minerals in Quartz-Monzonite-Porphyry from the 


Ryerson Mine . 3 

Orthoclase molecule . 16 

Albite molecule . 45 

Anorthite molecule . io 

Biotite . 6 

Quartz . 22 

Titanic iron ore . 1 


100 

From the thin section Lindgren concludes the most probable 
combination of the feldspars to give 38 per cent, albite and ortho¬ 
clase, and 24 per cent. Ab 2 An. Thus about 38 per cent, of the 
total feldspar is plagioclase. 


3 U. S. G. S. Prof. Paper 4 3 - 








538 


LOUIS E. REBER, JR. 


The system proposed by Iddings and Pirsson 3 for classifying 
rocks of this type on the basis of the nature of their feldspar 
content is shown graphically in the accompanying table. 

According to this classification a monzonite may contain from 
37^ to 50 per cent, of the total feldspar in the plagioclase form. 
As the rock under consideration contains about 38 per cent, 
plagioclase, it is clearly a monzonite porphyry on this basis, though 
not far removed from the granitic type. 

% % 

Alkali Feldspar. Plagioclase. 


Granite 

Monzonite . 
Granodiorite 

Diorite _ 


100 
, 8 7V2 
75 

62 y 2 

50 

2>7Y2 

25 

12*4 

o 


o 

I 2 l / 2 

25 

3772 

50 

62^ 

75 

87/2 

100 


The following secondary minerals are found in the monzonite 
and granite porphyries: sericite, quartz, pyrite, chalcopyrite, 
chlorite, chalcocite, kaolin, limonite, hematite, opal, cuprite, 
malachite, chrysocolla, and native copper. Sericite, quartz, 
chlorite, chalcopyrite, and pyrite are ascribed to hydrothermal 
alteration; chalcocite and kaolin to secondary enrichment: and 
kaolin with the remaining minerals to weathering. 

Diorite Porphyry .—Very fresh specimens of the diorite por¬ 
phyry are readily obtainable. Thus the opportunities for study 
of the unaltered rock is more favorable than in the case of the 
other phases of the porphyry of which even moderately fresh 
samples are obtained with difficulty. 

The typical fresh diorite porphyry is a somewhat flinty gray 
rock, showing numerous small white plagioclase phenocrysts in a 
fine dark green groundmass. Both the depth of the green color 
and the abundance of the phenocrysts vary somewhat in different 
parts of the mass. Portions of the diorite porphyry show a scat¬ 
tering of long needle-like hornblendes, up to 1 cm. in length, which 
3 As stated by L. V. Pirsson. 







MINERALIZATION AT CLIFTON-MORENCI. 


539 


are absent elsewhere. A scattering of tiny biotites is revealed by 
close scrutiny in nearly all the fresh material. In so far as this 
rock has been weathered or otherwise altered it tends to lose its 
green color. 

The hand specimens show the original ferro-magnesian min¬ 
erals of the diorite porphyry to have been hornblende and biotite. 
In thin section the hornblende is not easy to identify. It is prob¬ 
ably represented by aggregates of fine secondary green hornblende 
and green biotite masses. The original biotites are usually in part 
preserved imbedded in secondary chorite. In many specimens the 
ferro-magnesian minerals are rather abundant. Pyrite is found 
occasionally in the ferro-magnesian areas. The plagioclase pheno- 
crysts usually occupy about one half the area of the thin section 
and are of all sizes up to 4 mm. in diameter. They range from 
andesine to a rather basic labradorite, and the larger individuals 
are usually zonal. Epidote is a common alteration product and 
sometimes replaces entire phenocrysts. In some cases calcite and 
other carbonates may be abundant. Also flakes of colorless sec¬ 
ondary mica are rather common. Tiny magnetites are usually 
present. Very rarely quartz phenocrysts are observed. 

The groundmass varies from micro-granitic to trachytic in 
texture and probably contains some orthoclase a9 well as plagio¬ 
clase. The same alteration products occur in the groundmass as 
in the phenocrysts, but in the groundmass they are often difficult 
to identify. 

A partial analysis of the diorite porphyry from Prof. Paper 
43 gives Si 0 2 61.20; CaO, 5.11; Na 2 0 , 5.70; K 2 0 , 1.35, which 
is equivalent to 8.0 per cent, orthoclase molecule; 48.3 per cent, 
al'bite molecule; and 25.4 per cent, anorthite molecule. Thus it 
would appear that the plagioclase of the specimen analyzed was 
andesine. 

The alteration which has affected the diorite results in the de¬ 
velopment of calcite, epidote, chlorite, hornblende, sericite, green 
biotite, pyrite, and possibly a very little secondary quartz. On 
the basis of these minerals and the field relations this alteration 
is classed as hydrothermal. It is possible that some of the most 


540 


LOUIS E. REBER, JR. 


calcitic phases are due to weathering, though as abundant calcite 
is in most cases intimately associated with the fine white mica, 
this is not believed to be the case. 

Green Mica Rock .—The most characteristic, though probably 
not the most abundant phase of the green mica rock is uniformly 
fine grained and of a dull green color which appears black when 
moist. It contains abundant pyrite which occurs finely dissemi¬ 
nated and in small veinlets. Though having a dull appearance 
and a rather soft consistency, the crystalline nature of the rock 
is indicated by numerous minute cleavage faces which show on a 
fresh fracture. Sometimes apatite crystals show as tiny white 
hairs which give the rock a diabasic appearance. The different 
variations of this rock comprise finely mottled types in which 
white chalky material is mixed with the green material in various 
proportions, and various mixtures with brown feldspar and sili¬ 
ceous material. 

In the most typically developed phase this rock is made up of a 
fine felt of brownish green mica in which are imbedded numerous 
small rods of apatite, abundant pyrite, and more or less fine mag¬ 
netite (fig. 3). The pyrite contains microscopic quantities of 
chalcopyrite. The apatite rods are usually about .1 mm. in di¬ 
ameter and from .2 to .4 mm. in length (fig. 4). The magnetite 
is usually difficult to detect in the thin section, but a small amount 
can be separated from the powdered rock with a magnet. In 
some specimens, however, it is conspicuous and abundant. Py¬ 
rite masses occupy a fifth or a sixth of the section area (figs. 
3, 4, and 5). They are sometimes coated with a little chal- 
cocite. In some parts of the rock more or less nearly colorless 
chlorite is associated with the green mica. Where the pyrite is 
most abundant there is often a little quartz. 

The pyrite in some instances has a peculiar rounded branching 
form, somewhat similar to a portion of a micrographic inter¬ 
growth (fig. 4). Hematite often occurs with the magnetite, 
either massive or coating the other oxide. There are also grid¬ 
iron forms suggesting the form of rutile called sagenite, but evi- 


MINERALIZATION AT CLIFTON-MORENCI. 


541 


dently made up of plates rather than rods. These are believed 
to be hematite developed along cleavages of former pyroxenes. 
In the specimens similar gridirons of dark clayey material show 
124 0 angles corresponding to basal hornblende sections. Defi¬ 
nitely outlined areas, evidently preserving the form of previous 
amphiboles or pyroxenes, filled with clay, magnetite, pyrite, and 
hematite, occur with relative abundance. 

The apatite is the oldest of the minerals now present in the 
rock. It usually has fractures filled with the green mica, and is 
often included by pyrite. In rare instances the pyrites are broken 
and contain green mica in the fractures, indicating at least some 
green mica younger than the pyrite. 

The microscopic evidence shows that the original of the green 
mica rock was in its most extreme phase composed very largely 
of ferro-magnesian minerals, chiefly pyroxene or hornblende, 
with both probably represented, and sprinkled with abundant 
tiny apatites. It also contained more or less magnetite as an 
original constituent, and probably some feldspar. This extreme 
phase was evidently associated with gradations to the normal 
white porphyry. 

The minerals of secondary occurrence in this rock are: green 
biotite, pyrite, chalcopyrite, chlorite, magnetite, hematite, quartz, 
chalcocite, kaolin. 

With the exception of the chalcocite and kaolin, which are 
attributed to processes of secondary enrichment, the minerals are 
probably to be attributed to a single type of alteration. The 
intimate association of this rock with the micro-pegmatite sug¬ 
gests that this alteration may have contact metamorphic or pneu- 
matolytic affinities. However the similarity of the green biotite 
to sericite and its association with minor amounts of chlorite 
is most suggestive of that type of alteration classed as hydro- 
thermal. 

Micro-pegmatite .—-The micro-pegmatite is characterized by 
peculiar brown feldspar and inconspicuous gray quartz with abun¬ 
dant pyrite, some of which is in fine veins. The pyrite is less 
abundant than in the green mica rock. In some specimens the 


542 


LOUIS E. REBER, JR. 


EXPLANATION TO PLATE XX. 

Fig. 5. Green mica rock showing abundant pyrite and apatite. Ordinary 
light. 30 diameters. 

Fig. 6. Micropegmatite in contact with green mica rock. Crossed nicols. 
30 diameters. 

Fig. 7. Quartz-orthoclase micropegmatite. Crossed nicols. 30 diameters. 
Fig. 8. Diabase showing typical ophitic texture. Crossed nicols. 30 di¬ 
ameters. 


Plate XX. 


Economic Geology. Vol. XI 



Fig. 


Fig. 8. 




MINERALIZATION AT CLIFTON-MORENCI. 


543 


micro-pegmatitic structure can be recognized with a hand lens. 
The niicro-pregmatite is quite generally associated with the green 
mica rock which it intrudes on a minute scale (fig. 6). 

This rock when typically developed is very largely composed of 
a micrographic intergrowth of a rather irregular character com¬ 
posed of quartz and orthoclase feldspar (fig. 7). Sometimes 
the intergrowth is extremely coarse and sometimes very fine. A 
regular gradation from coarse to fine is not uncommon in a single 
individual. Large massive quartzes and orthoclases are also 
present. They sometimes occur in optical continuity with micro¬ 
graphic individuals. 

Linder the microscope the orthoclase where least altered has a 
dirty appearance like that produced by kaolinization and shows a 
distinct pale brown color when the thin section is observed with 
the unaided eye. Although the greater part of the orthoclase 
has been completely changed to sericite the preservation of con¬ 
siderable unsericitized feldspar in this rock in an area where no 
trace of original feldspar remains in the white porphyry suggests 
that the intrusion of the micro-pegmatite took place after the seri- 
citization was partially complete. However, the more siliceous 
character of this rock may be in part responsible for the preserva¬ 
tion of the orthoclase. 

The alteration products found in the micro-pegmatite are seri¬ 
cite, quartz, and pyrite, and perhaps the indeterminate brown pig¬ 
ment material found in the least altered orthoclase. The seri¬ 
cite, quartz, and pyrite are the work of hydrothermal metamor¬ 
phism. The brown pigment material may be a kaolinitic product, 
but it is believed unlikely that any . meteoric agencies are respon¬ 
sible for its development. 

Diabase. —The diabase is uniformly dark blue-gray to black in 
color. The texture ranges from that of a rather fine-grained 
variety to that of a variety which shows luster mottlings up to 5 
or 6 mm. in diameter. The luster mottling is due to the inter¬ 
rupted cleavage faces of the individual poikilitic augites. In 
some of the coarser-grained types the plagioclase laths are diffi¬ 
cultly visible. With alteration the diabase becomes lighter colored 



544 


LOUIS E. REBER, JR. 


and clayey in appearance. Where cupriferous it is full of irreg¬ 
ular quartz stringers with more or less chalcopyrite. 

Thin sections of this rock show a beautiful development of the 
diabasie or orphitic texture (fig. 8). Lath-like la'bradorite plagi- 
oclases form an interlacing mesh over the section like jackstraws. 
In the freshest specimens the greater part of the interstitial space 
is taken up by large nearly colorless augites which include a num¬ 
ber of the feldspar laths. In most of the specimens the augite 
has entirely altered to a green hornblende which is full of tiny 
specks of magnetite. The hornblende has streaks of higher bire¬ 
fringence and greater pleochroism which maybe due to inclusions 
of the brownish green biotite which occurs sparingly in the sec¬ 
tion as an alteration product. In addition to the augite and horn¬ 
blende as interstitial filling, there are a good many aggregates of 
fine magnetite intergrown with talc and surrounded by serpentine. 
Sometimes they are formed of magnetite and serpentine alone. 
These magnetitic spots undoubtedly represent original olivines, 
now entirely altered (fig. 9). The serpentine manifests a tend¬ 
ency to migrate away from the olivine which furnished the ma¬ 
terial for its formation, and often fills fractures in the broken 
plagioclases. There is also more or less chloritic material devel¬ 
oped in the ferro-magnesian areas, but it is not conspicuous. 

There are occasional magnetite individuals of a different nature 
from those formed from the olivine. These are large and mas¬ 
sive with clear-cut boundaries and appear to replace ferro-mag¬ 
nesian material and feldspar indiscriminately. This magnetite 
sometimes includes scraps of other secondary minerals, thus prov¬ 
ing it a product of the mineralization, and not an original con¬ 
stituent of the rock. There is some fine white mica developed in 
the plagioclase, and scraps of calcite are occasionally observed. 

In one section the alteration is chiefly to fibrous hornblende 
with spots of magnetite, and some epidote and a little quartz in 
the most distinct of the numerous tiny veins. The magnetite in¬ 
cludes secondary hornblende. Hornblende-quartz veins cut the 
magnetite and the epidote-quartz veins. The epidote-quartz 
veins cut the hornblende veins more often than they are cut by 


MINERALIZATION AT CLIFTON-MORENCI. 


545 


them. Thus three overlapping phases are shown: (i) Magne¬ 
tite; (2) hornblende with a little quartz, and (3) epidote with a 
little quartz. 

The minerals developed by processes of alteration in the diabase 
are green hornblende, fibrous hornblende, serpentine, magnetite, 
chlorite, talc, biotite, calcite, quartz, and epidote. 

As the diabase is believed to be the youngest intrusive in the 
area, contact metamorphism can hardly be called on to explain 
the development of any of the secondary minerals listed above. 
Hydrothermal alteration is thus held responsible for the entire 
list. 

Granite .—The typical pre-Cambrian granite is a moderately 
coarse-grained rock with decidedly pink feldspars, gray quartz, 
and occasional bluish black spots which resemble magnetite. 
Sometimes the feldspar lacks the pink color, and the quartz is less 
distinctly gray. In some localities fine-grained phases are en¬ 
countered. A relatively local phase exhibits fresh black pyrox¬ 
ene with gray feldspar. In one locality the abundant ferromag- 
nesian minerals make the rock resemble a gabbro. 

In mineralized areas the granite often shows pyrite, or more 
rarely, chalcopyrite. It may become a uniform or blotchy deep 
red or a nondescript more or less chalky gray in color, and is 
sometimes so silicified that the feldspars are no longer distin¬ 
guishable. 

The characteristic reds and browns of the weathered granite 
are a picturesque feature of the landscape, where it is exposed. 

The most usual type of the granite, which was called a mag¬ 
netite granite in the field, in thin section shows large quartz and 
orthoclase individuals with coarse granitic texture. There are 
occasional areas where the quartz is associated with tiny magne¬ 
tites, scraps of titanite and more or less leucoxene as well as con¬ 
siderable green biotite (fig. 10). Often the orthoclases are re¬ 
placed by large microperthitic intergrowths of orthoclase and 
alkali-plagioclase. Small zircons are not uncommon. In most 
cases the feldspar is considerably kaolinized and only slightly 
sericitized. Near the porphyry contact the granite may be domi- 


546 


LOUIS E. REBER, JR. 


EXPLANATION TO PLATE XXL 

Fig. 9. Diabase showing spots of magnetite and talc representing former 
olivine. Ordinary light. 30 diameters. 

Fig. 10. Magnetite, titanite, quartz, and green mica in area representing 
former ilmenite (?) Ordinary light. 30 diameters. 

Fig. 11. Banded epidote-quartz rock. Ordinary light. 30 diameters. 

Fig. 12. Sericitized quartzite. Crossed nicols. 30 diameters. 


Plate XXI. 


Economic Geology. Vol. XI. 



. ii. 


Fig 


Fig. 12. 

























































































































MINERALIZATION AT CLIFTON-MORENCI. 


54 7 


nantly sericitized or contain a great deal of secondary quartz. 
In this case more or less pyrite is usually present, and epidote and 
zoisite may occur in considerable abundance. Chalcocite and 
cuprite are sometimes found, and in a few cases the granite is 
mined as ore. 

The gabbro-like phase has a rather typical coarse, granitic tex¬ 
ture. The following minerals occur as primary constituents and in 
about the order of abundance named: alkali-plagioclase with fine 
albite twinning, orthoclase, nearly colorless augite, micrographic 
intergrowth of quartz and orthoclase, quartz, brown biotite, mag¬ 
netite, and apatite. 

Some of the orthoclase is quite thoroughly kaolinized. Else¬ 
where it contains both sericite and kaolin. There are thoroughly 
sericitized spots. The relatively fresh augite has a dirty altera¬ 
tion product along cleavage planes which makes it look like dial¬ 
lage. The augite is largely altered to hornblende which is itself 
locally altered to green biotite, sometimes to rutile, and possibly 
to magnetite. 

The following minerals are developed in the granite by processes 
of alteration: kaolin, leucoxene, cuprite, chalcocite, sericite, py¬ 
rite, quartz, rutile, green mica, magnetite, hornblende, titanite 
(?) epidote, and zoisite. Four kinds of alteration are probably 
represented: first, weathering, producing kaolin, cuprite, and 
quartz; second, the work of meteoric waters in the zone of de¬ 
oxidation, producing chalcocite, kaolin, and quartz; third, hydro- 
thermal alteration, producing sericite, green mica, pyrite, mag¬ 
netite, rutile, titanite, hornblende, leucoxene, and quartz; fourth, 
contact metamorphism producing epidote, zoisite, and perhaps 
hornblende. 

Shale. —The most argillaceous rocks of the Devonian forma¬ 
tion are very pure and uniform. The color ranges from gray to 
black, but in some cases becomes nearly white where blocks of 
shale are imbedded in the porphyry. Between the clay shale and 
material to be classed as shaly limestone, there are all gradations. 
The lower part of this formation is largely argillaceous limestone. 
In so far as these rocks are calcareous their alteration is similar 


548 


LOUIS E. REBER , JR. 


to that of the limestone. The most common alteration of the clay 
shales results in a paler-colored variety. They are largely com¬ 
posed of material which is microscopically indeterminate. Some¬ 
times flinty epidotic rocks are formed. Epidote may occur dis¬ 
seminated throughout the rock or in tiny veinlets associated 
with quartz. Often minor amounts of tremolite or sericite occur, 
and it is probable that the white shales are largely sericite of sub- 
microscopic grain. In these light-colored alteration products the 
coloring matter has become aggregated into tiny dark specks. 
The altered shales may contain occasional pyrite grains. 

The Cretaceous shales are siliceous flinty rocks which often 
have very little shaly parting. They are uniformly dark green to 
black in color, or strikingly banded in gray and green. The 
banded shales are entirely very fine quartz and epidote ; the 
coarser quartz bands with more epidote (fig. n). The epidote 
makes up a fourth of the rock. The green or black shales are 
similar to the clay shales of the Devonian formation, but contain 
abundant fine angular sand grains and are more epidotic. 

The alteration minerals of the shales are epidote, tremolite, 
sericite, pyrite, and quartz. The evidence as to the nature of the 
alteration producing these minerals is rather indeterminate. Epi¬ 
dote and tremolite are usually ascribed to contact alteration, while 
sericite is typical of hydrothermal. Epidote is also very charac¬ 
teristic of propylitization, but occurs under a wide range of 
conditions. 

Limestone .—Fresh specimens of the limestone formations can 
readily be obtained within a short distance of the most intensely 
mineralized areas. The Cambro-Ordovician limestone varies 
from a fine-grained shaly gray rock to a crystalline, blue-gray 
variety of relative purity. In general it contains no fossils or 
other clues as to origin. Peculiar pebbly or concretionary forms 
are sometimes observed on the weathered surface. 

The typical limestone of the Mississippian formation is a 
coarsely crystalline gray rock which often contains abundant 
crinoid stems and rarer coral or bryozoan remains. When freshly 
broken it emits a strong odor of hydrogen sulphide. This phase 


MINERALIZATION AT CLIFTON-MORENCI. 549 

is known as the gray cliff limestone because of its massive char¬ 
acter which makes it the conspicuous cliff-forming member of the 
sedimentary series. Other subordinate varieties are coarsely 
crystalline dark blue limestone and various finer-grained less pure 
phases. 

Chemical analyses show magnesia to occur rather abundantly 
in occasional beds. The quantity required to make the rock a 
dolomite is rarely approached in the Cambro-Ordovician lime¬ 
stone, but more frequently in the Mississippian formation. 

Near the porphyry the limestone sometimes becomes pure white 
in color and more or less coarsely crystalline. 

The most abundant end product of the alteration of the lime¬ 
stone is a massive vitreous rock composed entirely of brown gar¬ 
net. Sometimes the crystal forms of the garnet show, and there 
is a little intergrown quartz. Some intermediate forms occur, 
and calcite, diopside, chalcopyrite, sphalerite, and garnet is not an 
extremely unusual association. Massive magnetite associated 
with more or less lime silicate material is another complete end 
product, while bright green rocks composed largely of epidote 
sometimes occur. More or less tremolite and serpentine may be 
associated with the epidote. 

Very commonly the limestone is merely discolored, with nu¬ 
merous stringers and veinlets of pyrite. The rock may be of a 
somewhat bleached greenish color with irregular stripes and lines 
of darker color. 

Tremolite is especially common in the shaly varieties. Antig- 
orite and nearly colorless chlorite are of very general occurrence 
and may be uniformly disseminated or predominate in certain 
bands, while other bands are of coarsely crystalline calcite. Py¬ 
rite and magnetite sometimes occur in bunches. Hematite (spec¬ 
ulate) is often associated with the magnetite and may occur in 
wonderful platy arborescent forms. The secondary quartz is al¬ 
most always limited to tiny veinlets in association with pyrite or 
epidote. Antigorite also occurs characteristically in veinlets. 
Calcite occurs in veins and is sometimes associated with epidote. 

Siderite sometimes occurs in massive form, and cerussite has 
been found in at least one locality. 


5 50 


LOUIS E. REBER, JR. 


Oxidized phases are most typical of the limestone which has 
been heavily pyritized, as the acid waters from the gossan pene¬ 
trate the limestone with relative ease. Thus nearly massive yel¬ 
lowish phases and red-brown and buff more or less spongy ma¬ 
terial are not rare. This may contain pyrite preserved in the 
more siliceous portion, or may contain copper carbonates. 

The most abundant development of the carbonate copper min¬ 
erals, however, is usually in entirely decomposed, soft, iron- 
stained material. Here azurite occurs characteristically in nod¬ 
ules often with hollow centers lined with perfect crystals, mala¬ 
chite in more irregular forms, and rarer cuprite mixed through 
the spongy limonitic material. Often the oxides of manganese 
may give the decomposed limestone a nearly black color. Alunite 
occurs typically in large nodular masses in decomposed soft shaly 
limestone, close to a porphyry contact. 

The secondary minerals developed in the limestones are garnet, 
epidote, magnetite, tremolite, antigorite, chlorite, specularite, di- 
opside, chalcopyrite, pyrite, sphalerite, quartz, alunite, limonite, 
and other secondary oxides and carbonates. The oxides and car¬ 
bonates are clearly due to weathering processes. The occurrence 
of the others in the specimens offers very little basis for their 
classification beyond the fact that garnet and magnetite are most 
often massive and most rarely found in veins. All these latter 
minerals, with the exception of serpentine, chlorite, and alunite, 
are usually classified as typical of contact metamorphism, while 
the serpentine and chlorite are believed to be of hydrothermal 
origin. The alunite is probably due to the work of secondary 
enrichment solutions. 

Quartzite .—The Coronado quartzite varies from a coarse¬ 
grained, pebbly, well-cemented sandstone to an extremely fine¬ 
grained, vitreous quartzite. The finer-grained phases are not nec¬ 
essarily the most vitreous, though this is usually the case. The 
coarser-grained phases are most abundant, especially in the lower 
part of the formation. The coarser varieties in the lower hori¬ 
zons may be somewhat arkosic, otherwise, the rock is a very pure 
sandstone or quartzite, of a white or yellowish color. 


MINERALIZATION AT CLIFTON-MORENCI. 55 1 

Nearly all the quartzite contains enough pyrite so that surface 
weathering develops brilliant red and red-brown colors which 
make the outcrops of this formation picturesque features of the 
landscape. 

The typical quartzite is composed of quartz grains of a rather 
uniform size, the larger ones well rounded and the smaller with a 
tendency to angularity. In the mineralized area the interstices 
are filled with sericite which ranges in abundance from a very 
small amount to a third or more of the rock (fig. 12). The 
actual size of the grains varies in different localities. The seri- 
citic phases have a chalky appearance similar to that of the min¬ 
eralized porphyry. 

It is the very sericitic types whicn are best mineralized and 
seem to be most favorable to secondary enrichment. Dissemi¬ 
nated pyrite, more or less heavily coated with chalcocite may 
occur abundantly. The pyrite is intergrown with microscopic 
amounts of chalcopyrite. Masses of mineralized quartzite are 
cut by occasional small quartz-pyrite or chalcedony pyrite veins. 

The more vitreous material is usually made up of interlocking 
quartzes with only small amounts of sericite. Pyrite may occur 
imbedded in the quartz with clean-cut margins and good crystal 
form, but it is not, as a rule, very abundant in this phase. 

The quartzite and quartzitic sandstone which occurs interstrati- 
fied with the limestone is usually finer-grained than most of the 
Coronado formation. The arenaceous beds of the Cretaceous 
formation are pure fine-grained sandstone and not at all quart¬ 
zitic. This sandstone is made up of uniformly fine, angular and 
sub-angular quartz grains, cemented by calcite and chalcedony. 

The association of the most vitreous phase of the Coronado 
formation with the mineralized area suggests that much of the 
cementation has been due to the mineralizing solutions which were 
associated with the porphyry intrusive. It appears that in some 
cases the rock was silicified and in others sericitized. 

The alteration which has affected the quartzite has resulted in 
the introduction of sericite, quartz, pyrite, chalcopyrite, chalced¬ 
ony, and chalcocite, in a manner entirely comparable to the char- 



552 


LOUIS E. REBER, JR. 


acteristic alteration of the white porphyry. The chalcocite, and 
perhaps the chalcedony, is due to processes of secondary enrich¬ 
ment, while the other minerals are related to the hydrothermal 
alteration processes of the primary mineralization. 

PROCESSES OF ALTERATION. 

Contact Metamorphism. —Lindgren 4 distinguished between 
contact and hydrothermal metamorphism by limiting contact 
metamorphism to the vicinity of igneous contacts (in practice 
rather loosely), while hydrothermal is related to fissures which 
serve as trunk channels for the active thermal solutions. 

At Morenci, and perhaps more notably elsewhere, as at Ouray, 5 
the effects described as most typical of contact metamorphism 
sometimes occur at considerable distances from the igneous con¬ 
tacts and are related to fissures as well as to especially favorable 
sedimentary beds. In some instances effects like those of contact 
metamorphism are believed to have been produced by solutions 
which have altered the outer portion of the intrusive mass. 6 From 
this it is clear that solutions of some sort are instrumental in con¬ 
tact metamorphism. 

It would appear to require a considerable stretching of the 
original conception of contact metamorphism to include all these 
instances. A conception of the areal relation sufficiently broad 
for this includes hydrothermal alteration with equal propriety. 

Such a viewpoint is taken by Barrell, 7 who uses contact action 
as a general term which he divides into contact metamorphism 
and contact metasomatism, the former involving change of form 
alone, and the latter change of composition. Contact metaso¬ 
matism is then divided into pneumatolytic alteration and hydro- 
thermal alteration, depending on whether the solutions are acting 
at a temperature above or below the critical temperature of water 
(365° and 200 atmospheres). This division of metasomatism, 
as he notes, can only be used indirectly in practice. Hence for 

4 U. S. G. S. Prof. Paper 43, p. 124 (1905). 

5 Irving, J. D., U. S. G. S. Folio 153, 1907. 

6 Spurr, J. E., Econ. Geol., Vol. 7, No. 5 , P- 485, 1912. 

7 Barrell, J., U. S. G. S. Prof. Paper 57, p. 116 (1907). 


MINERALIZA TION A T CLIFTON-MORENCL 553 

field use his distinction is based on the nature of the minerals 
formed. 

Usage has refused to abide by this nice discrimination between 
metamorphism and metasomatism. Metamorphism is most often 
used as a general term for alterations (particularly of a con¬ 
structive kind) which may or may not involve metasomatic ac¬ 
tion. This difference from present custom has resulted in con¬ 
siderable confusion in connection with the interpretation of Pro¬ 
fessor Barrelbs results by later writers. He has been repeatedly 
quoted as supporting the view that material is never introduced 
during contact action, though the contrary is definitely indicated 
in the Marysville report. 8 

The term contact metamorphism is now usually applied to con¬ 
tact alteration producing heavy anhydrous silicate minerals, not¬ 
ably lime silicates, and corresponds to Barrell’s pneumatolytic 
alteration. This restriction of a term which is logically a general 
one has come into vogue through the lack of a satisfactory desig¬ 
nation for the more limited field. 

A consideration of the recorded instances of contact metamor¬ 
phism brings out in most striking manner the fact that the term 
is typically applied to the alteration of sedimentary rocks, most 
notably limestones. Lindgren 9 recognizes this fact in discussing 
contact metamorphism in his book on “ Mineral Deposits.” In a 
similar manner hydrothermal alteration is found to be character¬ 
istically associated with igneous rocks. 

Here, then, is a rather fundamental distinction which appears 
to have escaped the recognition it deserves. This immediately 
brings up the question as to whether or not differences in the 
nature and condition of the metamorphic agents have not been 
unduly emphasized at the expense of this significant difference 
in nature of the material acted upon. To what extent is it pos¬ 
sible that the same solutions may be responsible for contact meta¬ 
morphism and for hydrothermal alteration, and to what extent is 
the common assumption that the latter is a lower temperature 

s U. S. G. S. Prof. Paper 57, W- 

9 Lindgren, “ Mineral Deposits,” 1913 , P- 664 . 


554 


LOUIS E. REBER, JR. 


phase justified by unquestionable evidence? The prevalence of 
this opinion is, under the circumstances, a strong point in its 
favor. However, it is never amiss to inquire to what extent it 
is merely a matter of opinion. 

The points upon which this conclusion rests are first: contact 
metamorphism has been assumed to occur only close to contacts 
and to take place early in the cycle of intrusion and mineraliza¬ 
tion, at a time when the intruded rocks are supposed to be at a 
maximum temperature; while hydrothermal alteration is known 
to take place chiefly after the outer portion of the intrusive mass 
has solidified. Second: minute structural relations, indicate 
greater ability of the solutions of contact metamorphism to pene¬ 
trate the rocks independently of Assuring than that possessed by 
the solutions of hydrothermal alteration, thus suggesting pneu- 
matolytic affinities for contact metamorphism. Third: the min¬ 
erals characteristic of contact metamorphism are in general of 
the dense anhydrous character believed to be typical of high tem¬ 
peratures, and in some specific cases have been determined to 
require high temperatures for their formation, while the more 
hydrous minerals most typical of hydrothermal alteration are 
generally considered as having lower formation temperatures. 

The validity of the first assumption is entirely destroyed by 
the lack of close association of some contact metamorphic masses 
with igneous contacts and the discovery by Spurr 10 of an instance 
of contact metamorphic effects produced by solutions which passed 
through fissures in the already solidified outer shell of the in¬ 
trusive mass supplying the material. 

Aside from the distinction between vaporous and aqueous so¬ 
lutions it is known that the resistance to flow through capillary 
and sub-capillary openings decreases markedly with increase of 
temperature. Thus, in so far as contact alteration is more per¬ 
vasive than hydrothermal, higher temperatures are indicated. 
Differences in the material acted on must be kept in mind in order 
to make this comparison a fair one. The greater penetrability 
of the sediments as opposed to igneous rocks may obviate the ne- 
10 Spurr, J. E., Econ. Geol., Vol. 7, No. 5, 485, 1912. 


MINERALIZATION AT CLIFTON-MORENCL 


555 


cessity for fracturing or the fracturing may be merely more diffi¬ 
cult to detect. In the case of limestone the relatively greater 
solubility may dispense with the need for fracturing. 

The third consideration is of great weight as contact metamor¬ 
phism may almost be defined as the variety of alteration which 
produces heavy anhydrous minerals which require high tempera¬ 
tures for their formation. However it has not been conclusively 
proved that the minerals of hydrothermal alteration may not in 
some cases be formed at equally high temperatures, though it has 
been shown that in some cases they are probably formed at lower 
temperatures. 

In view of the foregoing considerations contact metamorphism 
maybe defined as a type of thermal metamorphism of sedimentary 
rocks, notably limestones, related areally to igneous contacts in a 
broad way and resulting in the development of heavy anhydrous 
silicates of the high temperature type, notably lime silicates. It 
is entirely possible that such metamorphism is in part pneumato- 
lytic, and in part the work of aqueous solutions. 

The minerals generally classed as typical of contact metamor¬ 
phism are lime-garnets, epidote, wollastonite, diopside, amphi- 
bole, vesuvianite, magnetite, specularite, and pyrite, chiefly in 
limestones ; and amphibole, epidote, feldspars, Tiotite, andalusite, 
staurolite, scapolite, and quartz, chiefly in shales. 11 

Of these the following are found in the metamorphosed sedi¬ 
mentary rocks at Morenci: andradite (calcium-iron garnet), epi¬ 
dote, diopside (rare), amphibole (tremolite), magnetite, specu¬ 
larite, and pyrite in the limestones, and amphibole (actinolite), 
epidote, and quartz in the shales. In addition: antigorite, com¬ 
mon serpentine, and sericite occur in the limestones, while the 
quartzite (or sandstone) contains abundant sericite. The latter 
minerals are most typical of hydrothermal metamorphism. In 
the case of the “ quartzite,” which may have been originally an 
arkosic sandstone, the sericitization is to be classed as typically 
hydrothermal rather than contact metamorphic. 

Evidence as to the paragenesis of the secondary minerals in 
11 Lindgren, W., “ Mineral Deposits,” p. 664, I 9 J 3 - 


556 


LOUIS E. REBER, JR. 


the limestone is rather indeterminate. Epidote, tremolite, and 
serpentine are cut by later quartz-pyrite veins. Calcite, epidote, 
and chlorite occur associated in indefinite veins. Antigorite 
veins cut pyritized residuals of relatively fresh limestone. Ser¬ 
pentine occurs between garnet grains in a manner suggesting con¬ 
temporaneous development. Thus epidote is to some extent 
younger than, as well as contemporaneous with the magnesian 
minerals, and there is a suggestion of contemporaneity of garnet 
and serpentine. 

In Prof. Paper 43 Lindgren describes the alteration along a 
mineralized fissure as follows: 

“ For a certain distance from the vein, ordinarily not more than a few 
inches or a few feet, the limestone is bleached and heavily impregnated 
with pyrite. A typical example is a dolomitic limestone from the Black 
Hawk No. 3 tunnel of the Shannon mine. The rock is grayish-green 
and partly altered containing besides much fine-grained calcite aggregates 
of colorless pyroxene and amphibole. This is cut by veinlets of coarse 
calcite, containing intergrown anhedrons of pyrite and magnetite, with 
a little chalcopyrite. Adjoining these veinlets and extending about half 
of their width are altered bands in which the limestone has been com¬ 
pletely converted to prisms of colorless amphibole, probably tremolite. 
One or two of the same prisms are also contained in the veinlet itself, 
which apparently is produced by filling. Thus the metasomatic change 
exerted by the waters flowing through this fissure consists in the con¬ 
version of dolomitic limestone to tremolite.” 

Previous contact metamorphism has developed pyroxenes and 
amphibole and a vein including calcite, magnetite, and amphibole, 
etc., has developed amphiboles along its margin. Whether the 
alteration along this vein is classed as hydrothermal or contact 
metamorphism, it furnishes evidence of the close relationship 
between the two phases. As already brought out, the distinction 
between these two types of alteration, based on relation to Assur¬ 
ing, does not seem to be fundamental as even garnetization is 
seen to be related to Assuring in some instances. 

The question of the importance of magmatic contribution in 
the development of the lime silicates characteristic of the contact 
metamorphism of limestones has been debated vigorously within 


MINERALIZATION AT CLIFTON-MORENCI. 


557 


the last few years. Instances have been brought forward which 
conclusively prove the truth to lie with the upholders of one side 
or the other, indiscriminately. 12 The conclusion is unavoidable 
that in specific instances both extremes are represented. A con¬ 
sideration of those occurrences most nearly corresponding to that 
at Morenci strongly favors the belief that the great lime-silicate 
masses of these contact zones have been developed with the aid of 
enormous accessions of material furnished by the intrusive mass. 13 

At Morenci the most conspicuous and abundant lime-silicate 
rock is composed of massive calcium-iron garnet. 

The four possibilities in a given instance of contact metamor¬ 
phism are (i) that the material present in the original rock may 
remain of the same chemical composition, (2) that various 
amounts of certain constituents may be eliminated, (3) that ma¬ 
terial may be added, and (4) that certain constituents may be 
removed, wholly or in part, and other constituents may be intro¬ 
duced. Probably examples of each of these cases exist some¬ 
where. 

In the passing from a^ relatively pure limestone to a nearly pure 
garnet rock, only two of these possibilities are under considera¬ 
tion. Either the change has taken place entirely through the eli¬ 
mination of material or some material has been added and some 
subtracted. The garnet rock is about two thirds silica and iron 
oxide by weight, while a series of analyses of the limestone from 
the Detroit Copper Co.’s quarry in the member of the Modoc for¬ 
mation which is most subject to replacement by garnet rock, 
shows the material other than calcium and magnesium carbonates 
to average less than 3 per cent. It would take between 20 and 
30 cubic feet of limestone to make one cubic foot of garnet rock 
if the change were produced entirely by the elimination of 
material. 

If only C 0 2 were eliminated and other material added, a vol¬ 
ume increase of about one half would be required. 

The structural relations at Morenci indicate beyond a reason- 

12 Leith, C. K., and Mead, W. J., “ Metamorphic Geology,” p. 140, 1915. 

13 Lindgren, W., “ Mineral Deposits,” 667, 1914- 





558 


LOUIS E. REBER, JR. 


able doubt that there has been no such great contraction of vol¬ 
ume as that required by the adherents of the theory that little 
or no material was added to the original calcareous rocks. As 
has been emphasized before, the intrusive contacts are everywhere 
sharp and angular with many engulfed blocks and no crumpling 
of the sedimentary beds. The bedding of the limestone is in 
many places clearly preserved in the structure of the secondary 
silicate rock. Individual beds which have proved most favor¬ 
able to alteration lie between other beds whose unaltered char¬ 
acter allows no uncertainty as to their undeformed condition. 
Also in passing along the bed containing garnet rock to fresh 
limestone, no evidence of vertical settling can be found. Similar 
considerations eliminate the possibility of marked increase of 
volume. Thus it seems in this case to be indisputable that the 
development of the enormous masses of garnet rock which exist 
at Morenci is the result of a volume for volume replacement such 
as has been emphasized again and again by Lindgren. 

Another corroborative fact is found in the occurrence of lime- 
silicate material identical with that of the replaced limestone as 
undoubted fissure filling. 

Lindgren 14 computes that since the change has taken place at 
constant volume for every cubic foot affected approximately 29 
pounds of CaO and 74 pounds of C 0 2 must have been carried 
away, and 83 pounds Si 0 2 and 74 pounds Fe 2 0 3 have been added. 

It is in connection with these lime-silicate contact metamor- 
phic rocks that the phenomenon of replacement is most charac¬ 
teristically developed. 

In his study of metasomatic processes Lindgren 15 brings out 
the fact that the substitution of one molecule for another is not 
the usual governing principle in replacement as was formerly 
believed to be the case. In opposition to the principle of mole¬ 
cule for molecule replacement, he established the principle of 
volume-for-volume replacement as being of most general applica¬ 
bility. The emphasis laid on this latter principle carries with it 

14 Lindgren, W.; U. S. G. S. Prof. Paper 43, p. 154, 1905. 

15 Lindgren, W., Econ. Geol., Vol. 7, No. 6, pp. 521-536, 1912. 


MINERALIZATION AT CLIFTON-MORENCI. 


559 


the implication that it is based on some physical-chemical law 
such as that believed to govern the former. In all probability 
this is not the case. 

From a physical-chemical point of view we may look upon any 
mineralizing solution as part of a system which is endeavoring to 
maintain equilibrium under changing conditions. The possibili¬ 
ties of variation of the external conditions are so great that it 
may be assumed that equilibrium is almost never maintained. 
The changes of temperature and pressure, and of the solid ma¬ 
terials in contact with a given portion of the solution due to flow 
through the vein channels, and the change of the composition of 
the solutions due to admixture of different solutions by the inter¬ 
secting of lines of flow, will in general maintain the unbalanced 
condition of the system. Similar considerations apply to pneu- 
matolytic emanations traversing sub-capillary passages. In the 
continuous endeavor to reach equilibrium, the solutions may be 
depositing one or more substances or dissolving one or more sub¬ 
stances, or depositing some and dissolving some at any given 
point. It is in the last instance that we have the phenomenon of 
replacement. 

As a general physical-chemical proposition there is no constant 
quantitative relation between the material deposited and the ma¬ 
terial taken into solution. In discussing this relation volumet- 
rically there are three possibilities to be considered; namely, more 
material may be deposited, or more dissolved, or equal volumes 
may be deposited and dissolved. Aside from the question of 
available space, the possibility of the third condition being ful¬ 
filled, rather than another, is as the magnitude of a point com¬ 
pared with a straight line of indefinite extent. However, in re¬ 
placement the available space imposes a limit on deposition. The 
tendency to deposit more than is dissolved can only result in a 
volume-for-volume exchange. That the other possibility, of less 
deposition than solution is sometimes realized, is an observed fact, 
and its rarity in replacement bodies (in so far as it is of uncom¬ 
mon occurrence) may be explained by the fact that solutions tend 
to move in the direction of decreasing temperature and pressure, 


560 


LOUIS E. REBER, JR. 


and decreasing temperature and pressure in general promotes 
deposition rather than solution. The increased solubility of the 
material ’being replaced, due to the pressure exerted by the crystal 
growth of the material being deposited, may aid volume-for-vol- 
ume replacement. 

Here then we have the “Law of Equal Volumes” as estab¬ 
lished by Lindgren as fundamental in the formation of replace¬ 
ment deposits. According to the foregoing conception it is not a 
melocular or a volumetric equivalence which is most fundamental 
in replacement—it is the continued endeavor to reach equilibrium 
of a complex physical-chemical system whose unbalanced state 
is being more or less continuously maintained. That juvenile 
solutions usually have a greater tendency to deposit than to dis¬ 
solve, due to fall of temperature and pressure, is sufficient ex¬ 
planation of the fact that the volume-for-volume relationship is 
commonly exhibited in replacement. 

In the light of the foregoing conception of replacement it 
seems probable that pressure conditions tending to close up open¬ 
ings as fast as they were formed would hinder the development 
of volume-for-volume replacement such as exhibited at Morenci. 
In that case the occurrence of the replacement is evidence of the 
absence of great pressure. 

Hydrothermal Alteration (Propylitigation) .—Kemp 16 says in 
regard to the original use of the term propylite: 

“ It was created for a series of rather coarsely crystalline or granitoid 
andesites, that are of early Tertiary age, and often have the dark sili¬ 
cates altered to secondary minerals. The name means ‘ before the gates/ 
and the significance was that coming just before the geological time of 
the true volcanics, yet resembling them, they deserved this distinction. 
It is now obsolete, and reasons for its special existence were long ago 
exploded, but having been employed on the Comstock lode it has passed 
into western usage.” 

Becker 17 in connection with his work on the Comstock lode 
showed that the so-called propylites were highly altered andesites 
and basalts, hence the term propylitic alteration. Since then the 

16 Kemp, J. F., “ Handbook of Igneous Rocks,” p. 41, 1899. 

17 Becker, G. F., U. S. G. S. Mono. B, p. 33, etc., 1862. 




MINERALIZATION AT CLIFTON-MORENCI. 561 

term has been used rather indiscriminately. Probably the defi¬ 
nition best in accord with usage is any alteration of igneous rocks 
in which hydrous magnesian alteration products are important 
and are associated with more or less epidote and carbonates. 18 

The above definition justifies the application of the term pro- 
pylitization to the alteration which has affected the diabase at 
Morenci. This alteration is characterized by the development of 
serpentine, hornblende, tremolite, epidote, magnetite, calcite, chlo¬ 
rite, biotite, talc, and sericite. Some of these minerals suggest 
higher temperature and closer affinity to contact metamorphism 
than in the case of alteration where chlorite predominates. 

In the Lake Superior region the Kewatin greenstones in gen¬ 
eral are abundantly chloritized, while hornblendic phases occur 
locally where there is a fairly close association with the later 
intrusives. 

Without comparative analyses it is impossible to make any 
accurate quantitative determination of the change in chemical 
composition produced by the alteration of the diabase. However, 
it is probable that the change has not been extreme, as the sec¬ 
ondary minerals appear to be very largely related to the com¬ 
position of the primary minerals. 

A general study of the relationship between propylitization and 
sericitization indicates that the chemical changes in this case are 
probably very similar to those which have affected the porphyry. 
Fragments of diabasic breccia along quartz chalcopyrite veins are 
entirely sericitized. 

{Sericitization) .—Sericitization is the almost universal expres¬ 
sion of the hydrothermal alteration of acidic igneous rocks. Thus 
it is a common feature of many of the mining camps of the west¬ 
ern United States. A set of specimens of sericitized porphyry 
from the various western mining camps would not differ greatly 
from a rock heap from any particular one. 

The granite and monzonite porphyries at Morenci have their 
feldspathic and ferro-magnesian constituents entirely changed to 
sericite over large areas where the mineralization is most intense. 

1* Compare Lindgren, W., “ Mineral Deposits,” p. 446, 1913. 


5 62 


LOUIS E. REBER, JR. 


This development of sericite is characteristically associated with 
more or less pyrite in disseminated grains and in veins with 
quartz. More or less secondary quartz may also be added to the 
rock, though in general this is not conspicuous, and in some cases 
original quartz has been removed. 

In the less extreme phase off this alteration the feldspar may be 
only partially changed to sericite, and the biotites merely bleached 
or altered to Chlorite. 

The less quartzitic portions of the large engulfed blocks of the 
Coronado formation which occur abundantly along the border of 
the porphyry mass in the town of Morenci are sericitized in a 
manner entirely comparable to that in which the adjoining por¬ 
phyry is affected. Sometimes as much as a third of the rock is 
composed of sericite, though no evidence has been found indicat¬ 
ing the replacement of the original quartz grains by sericite. 

The diorite porphyry is much less widely or intensely altered 
than the more acidic phases of tbe porphyry mass. All in all, the 
alteration of this rock is to be called sericitization only by cour¬ 
tesy. Just as the diorite porphyry is, in a measure, a type inter¬ 
mediate between the granite-monzonite porphyry and the diabase, 
so, aside from the question of intensity, the alteration of the dio¬ 
rite porphyry is, in a measure, intermediate between the sericiti¬ 
zation of the one and the propylitization of the other. 

Sericite, calcite and epidote occur more or less abundantly 
through the rock. Green biotite, secondary hornblende, tremo- 
lite, chlorite, and rare pyrite represent the former hornblendes. 
The original biotites are bleached and changed to chlorite around 
the margins. 

A comparison of the analyses of fresh and altered specimens of 
the monzonite porphyry given in Professor Paper 43 shows the 
chemical changes due to sericitization. By means of a computa¬ 
tion to introduce a corrective factor for any change of specific 
gravity involved, the material actually introduced or removed is 
expressed in terms of per cent, of the original rock mass, and 
shown graphically on Plate VI. (broken line number 4). This 
plate is explained further on. 


MINERALIZATION AT CLIFTON-MORENCI. 


563 


(Biotitization ).—It is believed that the occurrence of green bio- 
tite in a similar manner to that of sericite has not been previously 
described. The conditions under which it has here been formed 
are somewhat problematical. The analogous occurrence of this 
secondary green biotite and the sericite indicates hydrothermal 
development, while the intimate association with the younger 
micropegmatite intrusive suggests pneumatolytic associations. 

This alteration most needs be studied entirely from the point 
of view of the completely altered rock, as no unaltered equivalent 
has been found. The green mica rock consists very largely of a 
finely divided felt of brownish-green biotite which is very similar 
to the sericite felt developed in the porphyry. Scattered through 
this felt are numerous small apatites and abundant pyrite masses. 
The apatite is clearly the oldest mineral present. Sometimes there 
is also a little quartz with the pyrite, and a small quantity of color¬ 
less chlorite is intergrown with the biotite felt. Magnetite and 
specularite are rarely observed in thin section, and fine magnetite 
is nearly always detectable in the powdered rock. 

The following chemical analysis (I.) was made of the green 
biotite from this rock. The separation of the mica was accom¬ 
plished with a heavy solution, and an examination of the sample 
with a magnifying glass showed it to be of a high degree of purity, 
although extremely minute traces of pyrite escaped elimination. 

Column II. gives an average analysis for the Morenci sericite 



I. 

II. 

III. 

Si0 2 . 

45-54 

46.70 

— 1.16 

AI2O3. 

18.98 

35.50 

—16.62 

Fe 2 03. 

-52 

1.12 

.60 

FeO. 

7.17 

.17 

7.00 

MgO. 

10.79 


10.79 

CaO. 

2.36 


2.36 

Na 2 0. 

1.19 

.36 

.83 

k 2 o. 

7.67 

11-55 

- 3-88 

h 2 o. 

1.80 

S-oo 

- 3-20 

h 2 o. 

.07 




0.00 



MnO . 

1.01 


1.01 

Ti0 2 . 

3 -OI 


3 -oi 


IOO.II 

100.00 
























5 6 4 


LOUIS E. REBER, JR. 


as deduced by Lindgren 19 from the rock analyses. Column III. 
is the result obtained by subtracting the sericite from the biotite. 

The biotite seems to be a rather average biotite in most re¬ 
spects. The amount of ferrous iron is trifle unusual, and the 
lime is higher than is commonly found. The contrast between 
this normal biotite and the markedly hydrous white mica which 
is typical of the sericitization suggests that a temperature differ¬ 
ence in addition to difference in the material acted on may be 
required to explain the differences in the two resulting altera¬ 
tions. The presence of magnetite and specularite point in the 
same direction as well as the somewhat coarser habit of the green 
mica as opposed to the sericite. The sericite also shows a tend¬ 
ency to develop a coarser texture in the vicinity of the biotite. 
The biotite occurs in small veins much more abundantly than seri¬ 
cite ever does. There is then at least a strong suggestion of 
higher temperature conditions incident to the formation of the 
green biotite than those under which the sericite normally devel¬ 
ops. These are of course readily explained by the presence of the 
micropegmatite. 

INTERRELATION OF PROCESSES OF MINERALIZATION. 

Sericitization and Propylitization .—In the outlying portions of 
the district where porphyry and diabase occur together, the op¬ 
portunity for the study and comparison of the effects of mineral¬ 
izing solutions on the two rocks is particularly good. The re¬ 
lation between the sericitic and propylitic types of hydrothermal 
alteration is a question which has been discussed by various 
students of the rock alteration accomplished by mineralizing 
solutions. 

The sericitic alteration which is universal in the mineralized 
portions of the porphyry changes a rock consisting of plagioclase, 
quartz, and occasional biotite phenocrysts, and fine-grained gra¬ 
nitic groundmass of quartz, orthoclase, and alkalic plagioclase to 
a finely felted aggregate of quartz and sericite. The outlines of 
the phenocrysts are preserved to a greater or less degree by areas 

19 U. S. G. S. Prof. Paper 43, p. 170. 



MINERALIZATION AT CLIFT ON-M 0 REN CL 565 

of relatively pure sericite. The original quartz of the rock usu¬ 
ally remains unaltered, and considerable secondary quartz may 
be introduced. This is in general inconspicuous. In the highly 
sericitized rock no trace of biotite remains. Less thoroughly 
sericitized specimens in which the plagioclase phenocrysts are not 
sericitized beyond recognition always show the biotite bleached or 
more or less completely altered to chlorite. 

This development of chlorite is clearly related to the propy- 
litic type of alteration, and occurs here as a less intense phase of 
sericitization. 

The diabase dike rocks, the alteration of which is believed to be 
correctly classified as of the propylitic type, were originally com- 
.posed largely of labradorite and augite. In the more altered 
rocks the labradorite is not entirely gone, though it is somewhat 
sericitized and encroached on by the serpentine resulting from the 
alteration of the augite. The augite is changed to hornblende or 
more completely altered to actinolite and serpentine. 

The sericitic alteration of the porphyry dikes adjacent to the 
diabase dikes is of less intense character corresponding to that 
observed in the outskirts of the mineralized area in the main por¬ 
phyry mass, where the biotites are still recognizable. 

Study of analyses illustrating rock alteration of these types in 
other localities brings out more strongly the relation between the 
sericitic and propylitic changes suggested at Morenci. 

Of all the available pairs of analyses illustrating rock alteration 
with which the specific gravity of the fresh and unaltered rocks 
are given, thus making possible the quantitative computation of 
chemical changes, 20 the group showing a gain in potassa includes 
all those representing sericitic and propylitic alteration. Taken 
as a class there are no uniform differences between the sericitic 
and propylitic types. The chemical changes common to both 
these types of alteration are quite uniform. The circular diagram 
(fig. 30) showing graphically the losses and gains involved in 
each case brings out this fact strikingly. Marked gains in po¬ 
tassa and combined water, and a marked loss of soda, a marked 

20 Method used by Ransome in U. S. G. S. Prof. Paper 75, 1911. 


566 


LOUIS E. REBER, JR. 


FeS 


loss in lime, and a somewhat smaller, though equally constant 
loss of magnesia, are shown. 


Si02 



Fig. 30. Sericitization and Propylitization. (1) San Francisco Monzo- 
nite, sericitic (?); (2) San Francisco Monzonite, sericitic; (3) Hailey, Idaho 
Diorite, sericitic and propylitic; (4) Morenci Granite Porphyry or Monzo¬ 
nite, sericitic and propylitic; (5) Leadville White Porphyry, sericitic and pro¬ 
pylitic; (7) Rimini Monzonite, sericite tourmaline; (8) Willow Creek, Idaho, 
Grano-Diorite, sericitic and propylitic; (9) British Columbia Diorite, pro¬ 
pylitic; (10) Hauraki, New Zealand, Andesite, propylitic. 


The iron oxides also show rather constant loss aside from that 



























































MINERALIZATION AT CLIFTON-MORENCI. 


567 


introduced with the pyrite which is usually an accompaniment 
of this type of alteration. The introduction of C 0 2 in greater or 
less amount also appears to be the rule, though probably this is 
more true of propylitization than sericitization. The diagram also 
indicates that the changes in silica and alumina are not essential 
features of this alteration, as sometimes gains are shown and 
sometimes losses. The pairs of analyses plotted are about equally 
divided between those which show a loss of silica and a gain of 
silica. 

In the utilization of pairs of rock analyses to determine ma¬ 
terial contributed and abstracted by mineralizing solutions, it is 
necessary to know the specific gravities of the fresh and altered 
rocks represented by the analyses, and the volume change. It 
has been found, however, that as a rule there is practically no 
volume change. Where there has been volume change it should 
be possible to recognize it in the structural features or the devel¬ 
opment of porosity. Assuming, then, two analyses, one repre¬ 
senting an unaltered rock, and the other a rock which has been 
subjected to action of mineralizing solutions, with the specific 
gravity of each determined and a reasonable probability that no 
volume change has taken place, it is possible to determine quanti¬ 
tatively the chemical changes which have taken place with a fair 
degree of accuracy. In making these computations the two anal¬ 
yses are first recalculated to total exactly 100 per cent. Each 
analysis is then divided through by its specific gravity, giving 
grams of each constituent in 100 c.c. of each rock. Comparison 
of these two columns will show the quantitative changes which 
have taken place. If the one representing the fresh rock be sub¬ 
tracted from that representing the altered rock, the positive values 
represent material added by the mineralizing solutions, and the 
negative values represent material abstracted, in terms of grams 
per 100 c.c. of rock. Dividing this column through by the specific 
gravity of the fresh rock expresses the same quantities in terms 
of grams per 100 grams of fresh rock. It is these values which 
are used in plotting the diagrams showing graphically the work 
of the mineralizing solutions. The possibilities of error in such 


568 


LOUIS E. REBER, JR. 


determinations lie in (i) the difficulty of securing accurate deter¬ 
minations of specific gravity, as the specific gravity, to be ac¬ 
curate for this work, should include such porosity as may exist 
in either of the rocks under comparison, and (2) the necessary 
assumption that there has been no volume change, which may be 
more or less well founded. However, it is reasonably certain 
that any inaccuracy in the result attained in this way will not be 
sufficient to invalidate the conclusions drawn from them in con¬ 
nection with this discussion. 

In the diagram used for the graphic presentation of these re¬ 
sults the heavy circle represents the composition of the unaltered 
rock while radial distances measured by the concentric circles 
represent percentages. The losses are measured from the heavy 
circle toward the center, and the gains are measured outward. A 
separate radial line or spoke on the wheel is used for each con¬ 
stituent, and the points showing the individual losses or gains are 
joined by straight lines to emphasize their position. Thus in the 
plate each broken line threading the spokes of the wheel repre¬ 
sents an instance of rock alteration. 

This diagram brings out quite strongly the intimate relationship 
between the solutions producing alteration of the sericitic and the 
propylitic types respectively. If the solutions are similar the 
marked difference in the alteration products ascribed to these 
processes must be due to the different composition of the material 
acted on. 

It has been pointed out that alteration of the propylitic type is 
observed at Morenci in the ferro-magnesian minerals of the less 
intensely altered porphyry, and succumbs entirely to the sericitic 
alteration in the more intensely altered phases. Similar cases of 
propylitic changes developing at an early stage in the rock alter¬ 
ation are observed at Butte 21 and elsewhere. The chemical 
change required to eliminate the chlorite developed by sericitiza- 
tion is obviously the removal of magnesia which is apparently a 
universal feature of both sericitic and propylitic alteration. The 

21 W. J. Mead, personal communication; also various published descrip¬ 
tions. 


MINERALIZATION AT CLIFTON-M 0 REN CL 


569 


more intense and thorough the alteration, the more magnesia will 
be eliminated, and the more the alteration will tend to become 
sericitic rather than chloritic or propylitic. Where the original 
supply of magnesia is somewhat limited, intense alteration easily 
eliminates the chlorite; while in the ferro-magnesian rocks the 
sericitic stage is seldom reached. 

Contact Alteration and Hydrothermal Alteration .—According 
to Van Hise’s 22 classification of metamorphic changes those which 
involve the breaking down of rock minerals into simpler forms 
are classed as katamorphic, while those involving the production 
of complex forms are called anamorphic. The process of 
weathering produces the type examples of katamorphic changes, 
and this process involves oxidation, hydration, and carbonation, 
while the constructive anamorphic changes are deoxidation, de¬ 
hydration, and decarbonation, thus further distinguishing be¬ 
tween the two types of metamorphism. Contact alteration is 
anamorphic in its nature.' 

The changes produced by hydrothermal alteration are typically 
katamorphic, though to a much lesser degree than in the case of 
weathering. The minerals produced by hydrothermal alteration 
are in general somewhat more hydrous and include more car¬ 
bonates than the igneous rock minerals which they replace, and 
they are probably somewhat simpler in chemical structure. Thus 
hydrothermal alteration is intermediate between weathering and 
alteration of the type which develops complex anhydrous sili¬ 
cates, and would be expected to grade into these other types at 
either extreme. The change from highly crystalline anhydrous 
minerals to the simpler and more hydrous compounds developed 
by the process of weathering and sedimentation and back again 
to the more complex form by anamorphic process has been termed 
the metamorphic cycle . 23 

The close association in the metamorphic cycle of the minerals 
typically developed by contact metamorphism and original igne¬ 
ous rock minerals accounts for the slight effect of contact meta¬ 
morphism on fresh igneous rocks; while the katamorphosed na- 

22 U. S. G. S. Mon. 47, 1904. 

23 Leith, C. K., Journal of Geology, Vol. 15, p. 303, 1907. 


570 


LOUIS E. REBER, JR. 


ture of sedimentary material renders it particularly susceptible to 
contact alteration. On the other hand hydrothermal alteration 
which is, in a measure, katamorphic in its nature affects igneous 
rocks more conspicuously than sedimentary rocks. Further it is 
to be expected that hydrothermally altered igneous rocks are 
more susceptible to contact alteration and contact altered sedi¬ 
ments to hydrothermal alteration than the original rocks. 

From the foregoing conception it would appear that there is a 
very fundamental distinction between contact metamorphism and 
hydrothermal metamorphism indicated by the nature of the rocks 
chiefly affected by each type of alteration. The difficulty often 
experienced in discriminating between contact and hydrothermal 
alteration suspected of occurring together in the same rock indi¬ 
cates the absence of any break between the two phases and fur¬ 
ther that the change from solutions producing anamorphic effects 
to those producing katamorphic effects or vice versa, may , be a 
gradual one. The distinction, based on the material affected is 
important. 

In the most conspicuous and extreme variety of contact meta¬ 
morphism, the development of massive garnet, great quantities 
of silica and iron are supplied by the mineralizing solutions, and 
lime and magnesia are removed. The solutions typical of hydro- 
thermal metamorphism are seen to possess a remarkable unifor¬ 
mity as to material added and subtracted despite differences in 
material acted on. Iron, sulphur, potassa, water, and quite often 
silica are added, while soda, lime, and magnesia are removed. 
The most significant chemical distinction appears to rest on the 
association of introduction of potassa with hydrothermal alter¬ 
ation. 

The contact metamorphism which has been under discussion, 
characteristically alters sediments, involves heat, and may involve 
the addition and subtraction of certain substances; hydrothermal 
metamorphism alters igneous rocks, involves somewhat less heat, 
and the addition and subtraction of certain substances, plus addi¬ 
tion of potassa. Contact metamorphism is a constructive process; 
hydrothermal is one with destructive tendencies. 


MINERALIZATION AT CLIFTON-MORENCI. 


571 


ORIGIN OF MINERALIZING SOLUTIONS. 

The diorite porphyry occurring chiefly as laccoliths should, as 
Lindgren suggests, represent the original material of the intrusive 
which, because of its isolated position, solidified with little oppor¬ 
tunity for differentiation. Its relative lack of mineralization may 
be similarly explained. 

In the main mass of the stock with a connection maintained 
with the magma reservoir below, the opportunity for differenti¬ 
ation would be good. The large masses of green mica rock 
which rapidly become more extensive with depth represent the 
alteration of a basic differentiate of the porphyry stock. As this 
basic material separated out, probably by gravitative settling of 
the heavy minerals which crystallize first , 24 the upper portion of 
the mass must have become more and more acidic, and rich in 
mineralizers.. 

Two processes are important in the separation of the mineral¬ 
izing solutions which are originally dissolved in the magma. 
One is relief of pressure due to upward movement, the other is 
crystallization due to cooling. 

The common association of mineralizers with porphyries 
rather than abyssal rocks points to the importance of relief of 
pressure in this connection. 

The crystallization and segregation of the basic material would 
concentrate the mineralizers in the acidic portions of the magma. 
Sufficient relief of pressure would permit them to take a vapor¬ 
ous form and thus in all probability become immiscible with the 
liquid magma. These gas bubbles might rise through the magma 
with considerable speed. 

The changes would take place in the stock before the reservoir 
below was affected. 

The mineralizing solutions would, in general, be pneumatolytic 
where they leave the solidifying magma, as it is greatly above the 
critical temperature of water, and water is considered an essential 
constituent of these solutions. Their continuance in a vaporous 

24 Bowen, N. J., Jour . of Geol., Dec., 1915, Supp. 


5;2 


LOUIS E. REBER, JR. 


condition would be dependent on the temperature gradient from 
the still liquid magma through the surrounding rocks. As con¬ 
vection is vastly more effective than conduction in the outward 
transfer of heat, the time when the maximum temperature reaches 
a given point is a matter depending on the vicissitudes of each 
particular case. In so far as the solutions active at any given 
point are aqueous rather than vaporous, it would indicate their 
more or less distant source. 

At some early stage in the mineralization a solidified outer shell 
would be formed about the intrusive magma, and the mineraliz¬ 
ing solutions would pass through cracks in this shell. Owing to 
the slight susceptibility of fresh igneous rocks to the anamorphic 
changes of pneumatolytic alteration, any effects produced would 
be lost in the later hydrothermal alteration. Any vaporous solu¬ 
tions following hydrothermal alteration, however, might be ex¬ 
pected to produce conspicuous results. The fact that such evi¬ 
dence of contact alteration minerals developed in the intrusive 
rock is of rare occurrence would indicate that the time of contact 
metamorphism is in general before that of hydrothermal. 

The separating out of the mineralizing solutions is probably 
largely a gravitative effect. It is not surprising that the mineral¬ 
izers show a preference for the upper portions of the intrusive 
mass. Butler 25 has shown statistically that apically truncated 
stocks are most favorable to mineralization. At Morenci the 
mineralization is localized at the two points where the Paleozoic 
series has been preserved by down faulting. These points repre¬ 
sent the highest preserved portions of the porphyry stock. The 
greater fracturability of the sediments as opposed to the pre- 
Cambrian granite may also have influenced this localization. The 
lack of mineralization in the more deeply truncated stocks which 
has been brought out by Butler indicates that the final giving 
off of the mineralizing solutions is from the upper portions of the 
stock and not from the deeper seated underlying magma reser¬ 
voir. Otherwise-the lower portions of the stocks or surrounding 
sediments should show the effects of the passage of the solutions. 

25 Butler, B. S., Econ. Geol., Vol. io, No. 2, p. 101, 1915. 


MINERALIZATION AT CLIFTON-MORENCI. 


573 


Fracturing would be most intense at margins and pneumatolytic 
solutions would affect sediments if not the igneous rock, while 
if the solutions were in liquid form they would surely leave 
traces in the stock itself. 

This is somewhat different from the conception of Spurr 26 in 
his theory of ore-deposition. He considers the ore solution and 
the intrusive rock associated with the ore-deposits common differ¬ 
entiates of a deep-seated magma, which may reach their final 
resting place more or less independently of each other. He con¬ 
siders the adjacent intrusive incapable of differentiation, and un¬ 
important as a source of ore solutions. The evidence here out¬ 
lined points to a tendency of mineralizing solutions to become 
concentrated in the upper portions of an intrusive stock, coinci¬ 
dent with the concentration of more basic materials in deeper 
seated portions. This conception indicates the importance of 
geologically rapid differentiation in the nearby intrusive masses 
as opposed to differentiation in the deep-seated magma reservoir. 

That material originally dispersed through the magma of the 
deep-seated reservoir and ultimately given off as mineralizing so¬ 
lutions may have first become concentrated in the uprising apoph¬ 
yses is not improbable. However, when it is considered that 
many stocks undoubtedly have great vertical extent and become 
larger with depth, it seems reasonable to suppose the magma of 
the stock itself competent to supply a great bulk of mineralizing 
material where conditions are especially favorable to its separa¬ 
tion as seems to have been the case at Morenci. 

The writer is indebted to the officials of the Detroit Copper 
Co., of Morenci, Arizona, for making possible the collection of 
material and for helpful suggestions. 

26 Spurr, J. E., Econ. Geol., Vol. 2, No. 8, p. 781, 1907. 





















































. 





































































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